Evolution of Pineal Control of Endocrine Function in Lower Vertebrates

Pineal bodies, and other associated parietal structures, areremarkably varied among vertebrates These organs apparentlyassumed, in the primitive vertebrates, a principal role in integrationof photic information. The pineal photoreceptor cell seems tohave evolved into the secretory pinealocyte that is found inmost of the higher vertebrates. Along with the evolution ofthe photoreceptor element into the pinealocyte, there is a concomitantshift in the neural connection of the pineal organ. The pinealo-fugal,sensory innervation gives way to an autonomic, pinealo-petalmotor innervation. Thus, direct photosensitivity was supercededby indirect, optically-mediated control of the now secretorypineal gland. Even though pineal organs display such unusual plasticity anddiversity across groups, responsivenses to light remains a constantfeature. Photoperiod may modify the diurnallyrhythmic patternsof melatonin secretion across seasons and invoke appropriate"programs" which permit an animal to anticipate seasonal changes.Thus, melatonin may be a key molecule, attuned to photoperiodicity,which has been selected through evolution to effect adaptationto annual events.